During certification of disc drives, media defects are identified and their addresses are stored in a reserve part of the recording media in the form of a defect table. Upon power-on of the disc drive, the defect table is retrieved and stored in a buffer memory, thus enabling the drive electronics to skip over the defective sectors during disc drive operations.
As drive capacities continue to grow rapidly, the number of media defects also increase. Although more of the recording media can be set aside for storing the defect table, increasing the buffer memory for storing defect information is a very costly option. Currently, a scratch across N tracks in the radial direction of the disc will require N number of defect table entries to represent the defects. With the average tracks per inch (tpi) projected to hit 100,000 tpi, a quarter-inch radial scratch will require 25,000 defect entries to represent it. This creates a very large memory requirement for storing only the defect table.
The U.S. Pat. No. 5,212,677 issued to Shimote et al. on May 18, 1993, describes an apparatus for conducting visual inspection of surface defects. The apparatus groups neighboring defects into clusters and determines the size of each clusters. Although a method of identifying and storing information on cluster-like defects is taught, it does not provide a compressed description of scratch-like defects, particularly irregularly shaped defects or defects that run across a number of tracks.
Scratch-filling algorithms are often applied during drive certification to pad existing defects to prevent the growth of these defects. This involves marking good sectors neighboring the defective sectors as defective sectors. As a result, the final defect table after drive certification is usually bigger than the original defect table.
To cope with the growing number of defects in high-density disc drives, there is a need for a more compact method of storing the defect table.